Method and apparatus for introducing diagnostic pulses into an analog signal generated by an instrument

An instrument for use with a sensing element that generates a low-level process signal representative of the value of a physical variable of a process medium, said instrument comprising:

• (a) means for generating an analog signal from said low-level process signal, said analog signal having an amplitude representative of the value of said physical variable, said analog signal amplitude having a predetermined range defined by a lower limit and an upper limit;
• (b) means for detecting a fault condition in said sensing element;
• and
• (c) means for periodically changing said analog signal amplitude by a predetermined amplitude for as long as said fault condition exists, said predetermined amplitude such that said periodically changed analog signal amplitude lies within said predetermined range.

In an instrument circuit having an instrument operating in conjunction with a receiving device and a sensing element that generates a low-level process signal representative of the value of a physical variable of a process medium, a method for notifying said receiving device of a fault condition in said sensing element using an analog signal generated by said instrument from said low-level process signal, said analog signal having an amplitude representative of the value of said physical variable, said analog signal amplitude having a predetermined range defined by a lower limit and an upper limit, said method comprising the steps of:

• (a) detecting a fault condition in said instrument circuit;
• (b) periodically changing said analog signal amplitude by a predetermined amplitude for as long as said fault condition exists, said predetermined amplitude such that said periodically changed analog signal amplitude lies within said predetermined range;
• and
• (c) providing an indication of said predetermined amplitude occurring in said periodically changed analog signal amplitude at said receiving device.

An instrument circuit comprising:

• (a) a receiving device;
• (b) a sensing element that generates a low-level process signal representative of the value of a physical variable of a process medium;
• and
• (c) an instrument operating in conjunction with said receiving device and said sensing element, said instrument comprising:
• (i) means for generating an analog signal from said low-level process signal, said analog signal having an amplitude representative of the value of said physical variable, said analog signal amplitude having a predetermined range defined by a lower limit and an upper limit;
• (ii) means for detecting a fault condition in said sensing element;
• (iii) means for periodically changing said analog signal amplitude by a predetermined amplitude for as long as said fault condition exists, said predetermined amplitude such that said periodically changed analog signal amplitude lies within said predetermined range;
• and
• (iv) means for transmitting said analog signal to said receiving device;
• said receiving device comprising means for providing an indication of said predetermined amplitude occurring in said periodically changed analog signal amplitude.